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I have an 85 Baja 190 Sunsport with a 5.0L Mercruiser engine that I am rebuilding. The engine currently uses a Thunderbolt IV ignition system. While rebuilding the engine I am planning on installing a larger duration and lift cam, different intake manifold, and a 4 barrel carburetor. My question is with the installation of the performance parts, is the existing ignition system capable of maintaining the proper amount of advance thru out the rpm range? Or does the advance curve within the ignition module need to be change and if so how do I change it?
 

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Hates: Liver. Loves: Diesel
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Wow... I'm building an 83 Baja 190 Sunsport. What a coincidence :)

I'm learning the requirements for marine stuff, so I'm no expert, but unlike cars, boats have no vacuum advance. I would say it should be fine, and if it isn't, buy a tuning kit and swap for slightly earlier curves with lighter springs.

That stock thunderbolt probably has all of the curve in by 3000 or so. You might want to experiment with having it all in by 2500, but I wouldn't go much lower than that.

Be really careful with the cam selection. I'm sure you already know about it, but I've just gone through two months of a really fast learning curve if you want to chat about it.
 

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Thanks for the info on the ignition.Any additional information you could pass on pertaining to cam selection and other engine parts, it would be greatly appreciated.

What are your plans for your 83 sunsport?
 

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just remember, the carb needs to be a marine carb and it needs a spark arrestor. also if you change the ignition, get a marine distributor.
 

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Yup, marine distributor, starter, air intake, alternator, all a must. Cams are a bit of a trouble area. Just like in a car, the whole kit and kaboodle has to be tuned together.

-First concern is making enough torque in the low end to get on plane. I've seen 400-hp engines that couldn't lift a skier out of the water because they didn't make any big torque until 3000 rpm. Shoot for a nice smooth torque curve that starts to drop off after planing RPM.
-Second is overlap. If you have exhaust reversion from a lot of overlap, it can suck water back in the engine from the manifolds and boom... hydrolock.
-Third is peak power. Too much cam puts the power peak at too high an RPM. In a car its fine to occasionally rev a small block to 6000, but continuous operation like in a boat is a no-no. Shoot for 5000 rpm, 5500 rpm tops. More is OK on a race-style boat, but our SunSports weigh in at about 2600 with a person and some gas. Not too heavy, but certainly not light.

At the very least if you switch cams you'll have to switch props, but that's not a big deal and doesn't have to happen right away. Keep in mind that the same stock Mercruiser cam was used on every one of their small block engines from the 190-hp 283 all the way up to the 320-hp Mag 350. Its capable of easily supporting 260 in your 305 with the right heads and compression.

My 83 Sunsport is getting a 350 to replace the 305. I haven't settled on a cam yet, but I'm shooting for 300-340 hp at 5000. I have a Mercruiser cam ready to go in, but its a flat tappet and I might use this opportunity to go with a roller cam. Its an instant 60 lb-ft boost everywhere below 3000 RPMs so its hard to avoid the benefits of the roller. Certainly not necessary, but more fun definitely :)

Take a look at these screen caps of some dyno simulations. This is as close as I can get to matching the merc numbers so you can compare. They go in order from lowest to highest: 305/260hp, 350/300hp, then 350/320hp. They are all with the same Melling 22124 cam that is used by mercruiser, just different heads, compression, etc.

The second screen cap compares your 305 with a 305 and a bigger cam. You'll see that the power peaks outside of your RPM range and the low-end torque has taken a hit.
 

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Hates: Liver. Loves: Diesel
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So, if I may suggest... maybe a 350? I know it adds more money, but not much. You can buy a no-core-required marine shortblock for about $800; a long block for a little more. Then just swap your other goodies over to the new block. The carb will probably need a little jetting, but everything else should work fine. If you have the rear-riser exhaust manifolds, I might suggest buying new ones with the center riser. Major help with exhaust flow.

I have to upgrade stuff... my 305 got water in it then sat for 6 years. If you pull the dipstick its just rusty water :) I found a 4-bolt 350 with 18,000 miles on it for $250 so I lucked out. Still has crosshatches in the bores. I'm doing Vortec heads, 9:1 compression, and either the Mercruiser cam (300hp/350tq) or a Comp Roller grind (310hp/410tq)
 

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The Thunderbolt IV module will have the degrees of advance marked on it (ex. V8-20R). That module is for a V-8 and gives 20 deg. of advance. Full advance doesn't occur until approx. 5200 rpm. Advance on that module would be about 15 deg. @ 3500 rpm. The most popular module for V-8 performance applications, the V8-HP module has been discontinued by Mercruiser. The graph I'm posting should help you.

Barry
 

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As for your cam selection, if you keep the 305, keep duration @ .050 to 216 or less. For a 350, 225 @ .050. Lobe separation should be at least 112 deg., 114 deg would be better. I agree with curtis about torque. I have re cammed many marine engines that idled too high and/or lacked low rpm torque and had a problem coming up on plane. A high idle is rough on that Alpha drive too.
He is giving you good advice on upgrading to a 350. You will be happier and if/when you decide to sell the boat it will be easier to sell with the 350. Curtis you have done your research. Your combo sounds like a good one.

Barry
 

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Discussion Starter #10
Again thanks, for all of the great replies all of the info is greatly appreciated. As I begin to make final decisions on particular parts to run I will post them up here for everyone.

As for just replacing the 305 with a 350, I would love to but that is just not in the cards right now. With everything else that I am planning on the boat, the added expense of the new engine is not in the project budget. So this is what I have to use and I am just trying to get the most out of it for the money.
 

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I'm actually saving money by switching to a 350. If I assemble it myself, I can count on $600 in machine work for the lower end and another $200 for head work. In this case I was able to find a longblock that needed no machine work for $250. I did have the crank checked because I found a nick in a main bearing, but that was $30 to have it polished. So the block is getting tanked for $35, I bought a new Melling cam for $30, new lifters for $15, and of course I'll need gaskets, thermostat, and all the little things, but I should be $300 to the good by switching up.

:) Then I'm totally screwing the budget by doing a retro roller :confused: I'm not so smart.

BUT... cam for cam, you should be able to save a ton by swapping to a 350, or at least have it be exactly the same cost.
 
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